Autism appears to be on the rise. The U.S. Centers for Disease Control and Prevention reports that there are 20 times more cases of autism today than there were in the 1980s. This figure, from the Los Angeles Times, shows a 200% increase in California:

The rise in cases of autism led scientists to ask whether there was an actual increase in incidence or if we were just getting better at identifying it. The evidence seems to suggest that it’s (at least mostly) the latter. Said anthropologist Roy Richard Grinker: “Once we are primed to see something, we see it and wonder how we could have never seen it before.”

But how to explain disparities like this?

Often regional differences in health and mental health can be traced to heavier environmental toxin loads. In most of those cases, though, clusters of illness occur in poor and often disproportionately non-white neighborhoods. Autism clusters were happening in class-privileged places.

Sociologist Peter Bearman discovered that these clusters were the result of conversation. Class-privileged parents had the resources to get their child diagnosed, then they talked to other parents. Some of these parents would recognize the symptoms and take their child to the doctor and… voila… a cluster. “Living within 250 meters [of a child diagnosed with autism], reports the Los Angeles Times, boosted the chances by 42%, compared to living between 500 and 1,000 meters away.”